Dye sensitized photoconductive material

ABSTRACT

A PHOTOCONDUCTIVE MATERIAL FORMED FROM THE ADMIXTURE OF A FINE POWDER OF CADMIUM CARBONATE AND CADMIUM SULFIDE WITH A DYESTUFF CAPABLE OF ABSORBING RADIATION ENERGY AND TRANSMITTING IT TO SAID FINE POWDER AND WHEREIN SAID ADMIXTURE MAY ADDITIONALLY INCLUDE IODINE OR AN IODINE-CONTAINING COMPOUND.

28, 1972 KATSUO MAKINO ETAL DYE SENSIIIZED PHOTOCONDUCTIVE MATERIAL 2 Sheets-Sheet 1 Filed May 10, 1971 FIG. I

HALE- DECAY EXPOSURE TIME (SEC) FIG. 2

WAVE LENGTH (X) INVENTORS KATSUO MAKINO IWAO SAWATO BY fawn 01mm, lflon ,Zum Maqowc ATTORNEYS NOV. 28, 1972 KATSUQ MAKINO ETAL 3,704,123

DYE SENSIIIZED PHOTOCONDUCTIVE MATERIAL 2 Sheets-Sheet 1 Filed May 10, 1971 FIG. 3

WAVE LENGTH (A) FIG. 4

TIME (SEC) United States Patent 3,704,123 DYE SENSITIZED PHOTOCONDUCTIVE MATERIAL Katsno Makino and Iwao Sawato, Minami-Ashigara Machi, Japan, assignors to Fuji Photo Film Co., Ltd., Kanagawa, Japan Continuation-impart of abandoned application Ser. No. 720,935, Apr. 12, 1968. This application May 10, 1971, Ser. No. 142,081

Int. Cl. G03g 5/04 US. Cl. 96-1.6 16 Claims ABSTRACT OF THE DISCLOSURE A photoconductive material formed from the admixture of a fine powder of cadmium carbonate and cadmium sulfide with a dyestufi capable of absorbing radiation energy and transmitting it to said fine powder and wherein said admixture may additionally include iodine or an iodine-containing compound. 1

This application is a continuation-in-part of application Ser. No. 720,935, filed Apr. 12, 1968, and now abandoned.

BACKGROUND OF THE INVENTION The present invention relates to photoconductive materials, and more particularly to photoconductive materials possessing a high degree of inherent sensitivity and spectrosensitivity.

Although the photoconductive materials of this invention can be used extensively in many applications, they are especially applicable as electrophotographic sensitive materials and hence the invention will be discussed principally with reference to electrophotographic applications.

Various types of electrophotographic sensitive materials are known and can be broadly classified into two categories, vacuum-evaporation types and binder types. Metallic selenium sensitive layers formed by vacuumevaporation belongs to the former whereas sensitive layers made by coating a supporting material with a fine powder of photoconductive substances such as zinc oxide or cadmium sulfide dispersed in synthetic resin binder belongs to the latter. The present invention relates to photoconductve materials of the latter type.

Typical of the fine powdery photoconductive materials commonly used include zinc oxide and cadmium sulfide. Zinc oxide, however, is not suitable for electrophotography wherein light in the visible spectral field is used, since its inherent sensitivity falls within a range of 3750- 3900 A. in wavelength. Accordingly, it is necessary to extend it over the visible spectral field such as by the method described in US. Pat. 3,052,540 wherein a coloring agent is added to the components of the sensitive layer. The same applies to cadmium sulfide and a method of sensitizing it by coloring matter is reported in The Journal of Optical Society of America, vol. 46, p. 13 (published in 1956).

However, zinc oxide which has been sensitized by a coloring agent as described above is disadvantageous since it is insufficiently sensitive, requires a large degree of pre-exposure and is subject to fatigue. Its characteristics are therefore unstable making it difiicult to use. Similarly, a sensitive layer composed of a film of fine cadmium sulfide powder dispersed in a binder is also disadvantageous since it is characterized by reduced sensitivity thereby requiring a sharp increase of film thickness. If film thickness is increased to elevate the initial surface potential so as to obtain sufiicient electrostatic contrast to form an electrostatic latent image on a sensitive layer, the sensitivity is further reduced and the photoconductive current response speed is low.

SUMMARY OF THE INVENTION A highly sensitive electrophotographic material which requires only slight pre-exposure and which is characterized by negligible deterioration has now been provided by the physiochemical combination of CdS'nCdCO wherein n is a number from '1-5.

The photosensitive materials of the present invention are further characterized by high thermal stability, and can be used satisfactorily for both positive and negative charging, and are excellent in electrostatic contrast and high in photoelectric current response speed. Moreover, these compositions are simple to make and to process.

An additional aspect of this invention is that additives such as sulfur, selenium or their compounds or iodine or its compounds can be used with the foregoing photosensitive materials to further improve their characteristics.

According to the present invention there is provided a photosensitive material comprising cadmium carbonate and cadmium sulfide to which a dyestulf can be added for absorbing radiation energy and transmitting it to said cadmium carbonate and cadmium sulfide combinations. The compositions of this invention are more sensitive than the same compositions without the dyestuif and have an increased range of spectrosensitivity.

BRIEF DESCRIPTION OF THE DRAWINGS The attached drawings illustrate the characteristics of the photoconductive material of the present invention as compared with photoconductive materal not sensitized by coloring matter.

FIG. 1 shows the relationship between the sensitivity of photoconductive material (in terms of half-decay exposure time) and initial surface potential,

FIGS. 2. and 3 indicate the spectral sensitivity of photoconductive material, and

FIG. 4 shows the changes in attenuation of surface potential of charged photoconductive material in dark as 'well as during exposure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The exact structure of the fine powdery photoconductive material of the present invention which comprises cadmium carbonate and cadmium sulfide as its main constituents is unknown, but it is known that it is not a simple mixture of carbonate and sulfide. Said photoconductive material can be prepared by simultaneously dropping a sulfur ion and a carbonic acid ion into an aqueous solution of water soluble cadmium salt or by adding sulfur ion to the suspension in which fine powders of cadmium carbonate are precipitated so as to convert part of the carbonate into cadmium sulfide. Cadmium salts, such as the halides, the sulfates or the nitrates, can be used and carbonates such as sodium carbonate, potassium carbonate and ammonium carbonate, can be used. A few examples of manufacturing process will now be described in the following.

MANUFACTURING EXAMPLE 1 Liquid A comprising 212 g. of sodium carbonate dissolved in 1.5 l. of distilled Water, liquid B comprising 457 g. of cadmium chloride (2% chloride) dissolved in 1.0 l. of distilled Water and liquid C comprising 78.1 g. of sodium sulfide anhydride dissolved in 0.2 l. of distilled water were prepared respectively.

250 g. of fine silica powder Aerosil (trademark: made by Degus Co. of West Germany) was added to liquid A, so as to form a suspension of the powder in the liquid. Liquid B was dropped slowly into this suspension to produce a white precipitate of cadmium carbonate. Liquid C was then added and part of the cadmium carbonate was converted into cadmium sulfide.

After fully washing, a yellow percipitate was produced which was dried at a temperature of 70 C. for about 30 hours. Subsequently, a low temperature burning treatment was applied at 200 C. for about 24 hours. The fine powders obtained by this method were usable as basic powders for the present invention.

MANUFACTURING EXAMPLE 2 Fine powders of cadmium carbonate and fine powders of sulfur 0.1-0.2,U. in particle diameter were mixed (at a mixing ratio of 70:30 by weight) and heated at some 450 C., whereby fine powders composed of cadmium sulfide and cadmium carbonate could be obtained. As a result of X-ray diffraction, it was found that cadmium sulfide was existing in hexagonal and cubic forms within the amorphous cadmium carbonate component. A composition analysis revealed the composition to be CdS-dCO The characteristic of photoconductive material under the present invention differs greatly from the characteristic of fine powders composed of cadmium sulfide only. The existence of cadmium carbonate in the former is important. That is, cadmium carbonate is white and possesses no absorption bands in the visible field. Accordingly the light absorption rate in the visible field of a photoconductive material consisting of cadmium carbonate and cadmium sulfide is, on an average, lower than that of a photoconductive material composed of sulfide only. For instance, when the photoconductive material according to the present invention was dispersed in a binder and a layer formed, the light absorption rate of said layer was found to be lower than that of materials not containing cadmium carbonate. This means that light penetrates deeply into the interior without being absorbed completely at the surface.

An electrophotographic sensitive layer made from fine photoconductive powders comprising cadmium sulfide only, active light which is capable of contributing to photoconductivity, is entirely absorbed in or near the surface and does penetrate deeply into the layer. The penetratable depth is about lit at the most. Accordingly, if the surface of the electrophotographic sensitive layer is charged and exposed, as in most electrophotographic processes, active light is absorbed in the surface layer to a thickness of 1,u at the most and it is necessary for the free charge carriers to travel to the opposite pole through the inner layers where no active light exists at all. Generally, the thickness of such inner layer is several tens of microns to about 100 microns and it is quite difficult for the carriers to travel over this distance without being caught in the sensitive layer composed of the fine photoconductive powders dispersed in a binder. On the other hand, when using a sensitive layer formed by dispersing fine photoconductive powders comprising cadmium carbonate and cadmium sulfide in a binder, active light tends to penetrate into the interior of the sensitive layer and excitation of the free charge carriers takes place in the inner layers as well. It is therefore presumed that free charge carriers can travel more easily. This is considered to be the reason why the film thickness can be increased without greatly sacrificing sensitivity. One may be inclined to expect the same effect of fine cadmium carbonate powders and fine cadmium sulfide powders dispersed in a binder, but it is not the case. That is to say, in such a sensitive layer, fine cadmium carbonate powders push themselves among fine cadmium sulfide powders and impede electrical interconnection of fine cadmium sulfide powders so that the expected photosensitivity cannot be secured. On the other hand, in the case of fine particles consisting of cadmium carbonate and cadmium sulfide, whose structures are such that the surface layers of cadmium carbonate particles are sulfurized to cadmium sulfide, electric interconnection is not obstructed thereby improving the light absorption rate.

Further, the photoconductive response speed of fine powders consisting of cadmium sulfide and cadmium carbonate is higher than that of fine powders composed of cadmium sulfide only. Though it is known from X-ray diffraction that fine powders composed only of cadmium sulfide form relatively fine crystals, cadmium sulfide of fine powders composed of cadmium carbonate and cadmium sulfide are remarkably crystallographically disturbed. Such disturbance of the crystal and lattice defects are considered to result from increased photoconductive response speed.

As above, fine particles consisting of cadmium sulfide and cadmium carbonate are used as a basic material in the present invention. Other materials may be added to these fine powders to an extent which will not impair their photoconductivity. For instance, in forming the precipitates of fine particles composed of cadmium sulfide and cadmium carbonate, it is possible to mix bentonite, fine silica powders or diatomic earth, etc. in the reaction system, thus increasing the volume of fine powders produced and preventing caking in the course of the subsequent drying process. These additives also act to some extent to aid the introduction of light.

To the base powders, comprising cadmium carbonate and cadmium sulfide, expressed as CdS-nCdCO wherein n is preferably less than 5, may be added appropriate components as required to increase photoconductivity. That is, fine powders of inorganic or organic compounds not involving light absorption in the spectral sensitivity region of cadmium sulfide may be added. As shown in the preceding example, fine silica powders are effective. Diatomic earth, zinc oxide, zinc sulfide, titanium oxide, aluminum oxide and magnesium oxide, etc., can also be added. Also, part of the sulfur of the cadmium sulfide may be substituted by selenium.

As described above, materials made by adding iodine or iodide to the abovementioned photoconductive mate- .rial are also desirable base powders applicable under the present invention and these photoconductive materials will be discussed in the following example.

MANUFACTURING EXAMPLE 3 Powder particles (CdS-15CdCO composed of cadmium sulfide and cadmium carbonate were dispersed in ethyl alcohol. An ethyl alcohol solution of iodide (1000 ml. of ethyl alcohol containing 40 g. of iodine) and iodine was added while stirring which was adsorbed by the cadmium sulfide-cadmium carbonate particles. After setting for several hours and removing the supernatant liquid, it was dried in a vacuum. Fine powders thus obtained were satisfactory as the photoconductive materials according to the present invention.

Iodides which can be added include the metallic iodides such as lithium iodide, magnesium iodide, beryllium iodide, bismuth iodide, tungsten iodide, cesium iodide, strontium iodide, tin iodide, potassium iodide, cadmium iodide, antimony iodide, aluminum iodide, zinc iodide and the like. They are all soluble in water or organic solvents. One of the most convenient modes of application is to bring the solution of these compounds into contact with the abovementioned photoconductive materials. It is also possible to expose the powders to the vapor of these compounds by surface adsorption of the powders or by dispersion in their interiors. On the other hand, in the case of iodides which are insoluble in both water and organic solvents, for instance, lead iodide, it is possible to vaporize the iodide and to bake the powder in the vapor.

Dyestuffs to be employed in obtaining the photoconductive material of the present invention should be such, as described above, which increases the inherent sensitivity of the fine powders or which adds and extends the spectro-sensitivity range. A great number of such dyes are available, that is, phthalein dyestutf (Eosine Rose Bengal, Fluoresceine, Froxine, Ethyleosine), triphenylmethane dyestulf (Malachite Green, Crystal Violet, Brilliant Green), cyanine dyestutf (dicyanine, criptocyanine, pinacyanol, neocyanine, melocyanine) and others (Rhodamine B, Methylene Blue), etc. These dyes are added to fine powders comprising cadmium sulfide and cadmium carbonate. More than two kinds of dye may be applied in an appropriate combination.

The dye adding method is the same as in the case of abovemeutioned iodide. In the case of water soluble dyestuffs, a slurry of the base powders is formed with an aqeuous solution of the dyestuffs. The slurry is dried and dispersed in a binder. The same addition procedure applies to the addition of dyes which are soluble in organic solvents. In short, dye is added together with iodine or iodide as required to the base powders and dry powders thus prepared, which are dispersed in a binder, or dye, together with base powders. Iodine or iodide is added to the binder, as required.

The photoconductive materials of the present invention are quite useful in that they are applicable not only as the sensitive materials in electrophotography, but also as the photoelectric conversion materials for photocell, photoamplifier, or X-ray/visible photo-image converter, etc.

The present invention will further be explained in detail with reference to some examples of practical application.

EXAMPLE 1 150 g. of base powder obtained in the same manner as described in Manufacturing Example 1 and not containing iodine were placed in the solution comprising 150 mg. of Crystal Violet dissolved in 150 1. of ethyl alcohol, stirred fully and dried. This is hereinafter called crystal violet dyed base powder.

Next, 60 g. of thermo-setting acrylic resin (containing 50% of solid), 50 ml. of thinner for said resin and 60 g. of crystal violet dyed base powder were kneaded for about 15 hours in a porcelain ball mill, fully mixed and dispersed to prepare sensitive paint, which was applied to a thin aluminum sheet to a thickness of approximately 5 to loop. (the thickness was measured after baking as mentioned hereinafter). Electrophotographic sensitive material was prepared by drying and baking for 30 minutes at 150 C. to harden the thermosetting acrylic resin binder. This is called here Sample B COMPARATIVE EXAMPLE 1 To 240 g. of base powder as obtained in the abovementioned Manufacturing Example and not containing iodine were added 500 ml. of ethyl alcohol solution containing 55 g. of dissolved cadmium iodide, which was fully stirred and dried. This will hereinafter be called base powder containing iodine. Electrophotographic sensitive material was made using these fine powders in the same manner as described in Example 1. This is called here Sample C.

EXAMPLE 2 Powder containing iodine was prepared as shown in the Comparative Example 1 and, using this powder, a base powder was prepared in the same manner as in the case of Example 1 and electrophotographic sensitive material was made using this fine powder similarly to the aforesaid example. This is called here Sample C The effect of the present invention will now be described by comparing the following samples: Sample A (comparative sample)=fine powders of pure cadmium sulfide; Sample B (comparative sample, according to the Comparative Example 1)=fine powders of (cadmium sulfideH-cadmium carbonate); Sample B (according to Example 1)=fine powders of (cadmium sulfide+cadmium carbonate+coloring matter); Sample C (comparative sample according to Comparative Example 1)=fine powders of (cadmium sulfide-l-cadmium carbonate-Fiodine); sample C (according to Example 2)=fine powders of (cadmium sulfide+cadmium carbonate-i-iodine-lcoloring matter).

The above samples were charged electrostatically by means of a corona discharge device impressed with DC -7.0 kv. The surface potential several minutes after charging (called initial surface potential) was measured. Next, the sensitive materials were illuminated by a tungsten-filament lamp having a color temperature of 2666" K. and a surface intensity illumination of 15 luxes. The time required for the surface potential to be decayed to half (called half-decay exposure time) was measured. FIG. 1 illustrates the relationship between initial surface potential and half-decay exposure time in respect to these samples.

In FIG. 1, the abscissa is graduated logarithmically to indicate half-decay exposure time (Unit: second) and the ordinate is graduated logarithmically to indicate initial surface potential (Unit: volt). In this figure, the curves A, B, B C and C have been established from the measured values of specimens A, B, B C and C The following considerations were made on the basis of the curves shown in FIG. 1. That is, initial surface potential increases with the film thickness of the sensitive layer and is considered to be roughly proportional to the film thickness generally in the case of less than 1000 v. Accordingly, if the ordinate (initial surface potential) of FIG. 1 is read as the film thickness, it is found that in the case of sample A, the half-decay exposure time is extremely dependent on film thickness. In the case of Sample B and B such dependence on film thickness somewhat decreases while Sample C and C containing cadmium iodide scarcely depend on the film thickness. Sample B contains crystal violet dyed base powder, but possesses generally many of the characteristics of Sample B. As a result, it rather showed a tendency of increased dependence on the film thickness. The sensitive material containing both cadmium iodide and crystal violet shows the highest sensitivity, the excellent effect of co-existence of both being thus demonstrated in FIG. 1.

EXAMPLE 3 In the same manner as in the case of Example 1, g. of base powder containing cadmium iodide, to which 15 mg. and 150 mg. of crystal violet were added, were prepared respectively. The latter showed sensitivity about twice that of the former. Like this, the rate of increase in sensitivity varies with the amount of coloring matter added.

EXAMPLE 4 There is no prescribed system of indicating the photosensitivity of electrophotographic sensitive material and, at present, various systems are employed. As described above, (cadmium sulfide+cadmium carbonate) sensitive materials according to this invention possess the characteristics that (1) the initial surface potential increases with film thickness and (2) photosensitivity depends on film thickness. Accordingly, sensitive material, having different film thickness so that the initial surface potentials were 1000 v., were measured as to their sensitivity under varying exposure conditions, expressed in half-decay exposure time. The results are shown in the following table. Each of the sensitive materials were prepared as described in Example 1.

It can be seen from Table 1 that the addition of coloring matter does not necessarily bring about increased sensitivity. For instance, in the case of Brilliant Green, if cadmium iodide is not present, addition of coloring matter brings about little effect whereas if it is present, sensitivity is increased 17 times. The table also shows that there are certain types of coloring matter which bring about a sensitization effect even if cadmium iodide were not present.

EXAMPLE 5 Ethyl alcohol solution of parts by weight of iodine was added to 100 parts by weight of the previously prepared base powder comprising CdS and CdCO' to make a slurry. The slurry was dried for 20 hours at 70 C. and a base powder containing iodine thus recovered. In the same manner, coloring matter was added. The volume added was 0.15 part by weight as against 150 parts by weight of base powder containing iodine. To 100 parts by weight of dyed photoconductive powder thus obtained was added 50 parts by weight (solid content) of thermosetting acrylic resin paint which was dispersed uniformly therein so as to make sensitive paint. This paint was applied to an aluminum plate and baked for 30 minutes at 150 C. so as to obtain an electrophotographic sensitive material. The surface of the sensitive material was charged negatively and the following values were obtained as a result of the measurement of photosensitivity as represented by the half-decay exposure time of the surface potential caused by illumination intensity of luxes.

The following example shows how spectral sensitivity varies depending on the addition of coloring matter.

EXAMPLE 6 Similar to Example 1, the following samples were prepared. That is, Sample A=fine powders of pure cadmium sulfide, as a comparative sample; Sample C=fine powders of (cadmium sulfide+cadmium carbonate-l-cadmium 10- dide) were prepared and further Sample C =comparative Sample C with Rose Bengal added thereto. Sample C =the same with Eosine Y added thereto, Sample C =the same with Bromophenol Blue added thereto, Sample C ==the same with Crystal Violet added thereto, Sample C =the same with Malachite Green added thereto, Sample C =the same with Brilliant Green added thereto, Sample C =the same with Rhodamine B Conc. added thereto, were prepared. These dyed electrophotographic sensitive materials were charged electrostatically by means of a -6.0 kv. corona discharge device. They were exposed by means of a diffraction grating type spectrograph equipped with tungsten-filament lamp as the light source, and subsequently developed. The outline of spectrographic images of spectral sensitivity is shown in FIG. 2. In FIG. 2, the ordinate is graduated in relative values of the reciprocal of exposure and the abscissa in wavelength (Unit: A). The characteristic of tungsten-filament lamp is not compensated in FIG. 2.

As will be seen from FIG. 2, the range of sensitivity of sensitive material has been greatly enlarged by the addition of coloring matter. The inherent sensitivity was thereby increased. Table 2 shows the specific exposure required to obtain the same sensitivity at the wavelength 5000 A. All the results in Table 2 were obtained by dividing the exposure of samples subjected to sensitizing treatment (addition of dyestuff) by the exposure of samples not subjected to sensitizing treatment.

TABLE 2 Samples Specific exposure Rosen Bengal 0.67 Eosine Y 0.4 Bromophenol Blue 0.66 Crystal Violet 0.45 Malachite Green 0.60 Brilliant Green 0.40 Rhodamine B Conc 0.80

EXAMPLE 7 Fine photoconductive powders prepared in the preceding example and composed of CdS and CdCO were heattreated after addition of cadmium iodide as follows, and photoconductive base powders prior to addition of coloring matter were prepared. To 500 g. of fine photoconductive powder comprising CdS and CdCO were added g. of cadmium iodide dissolved in 50 ml. of ethyl alcohol to make slurry, which was then agitated fully. The mixture was dried for about 36 hours at 70 C. and further heat-treated for about 36 hours at 200 C. To 100 g. of the thus obtained base powder was added 100 mg. of Malachite Green and 100 mg. of Rhodamine B Cone. dissolved in 100 ml. of ethyl alcohol to make slurry. The slurry was stirred fully and dried for about 36 hours at 70 C., dyed photoconductive powders being thus obtained. In the same manner as in the preceding example, these powders were dispersed in a thermosetting acrylic resin to make a photosensitive paint, which was then applied to an aluminum plate and baked for 30 minutes at C. An electrophotographic sensitive material was thus obtained. The spectre-sensitivity of this material was equivalent to the combination of spectrosensitivity obtained when both types of coloring matter were applied independently, as illustrated in FIG. 3. In this figure the curve C is the spectre-sensitivity curve of the sample used in this example while the curves C and C are the spectro-sensitivity curves respectively obtained where Malachite Green and Rhodamine B Conc. were applied independently.

The improvement of photoelectric current response speed by means of addition of dyestuif will now be explained by example.

EXAMPLE 8 Electrophotographic sensitive material sensitized by coloring matter as obtained in the preceding example was electrostatically charged by means of a corona discharge device. It was exposed by using a tungsten-filament lamp as the light source and the surface potential in the dark after charging and the surface potential during exposure were continuously measured and recorded. FIG. 4 illustrates the result, with the ordinate being graduated logarithmically to indicate surface potential (Unit: volt) and the abscissa indicating the time (Unit: second). Exposure was commenced at time zero on the abscissa, with the changes in surface potential in the dark being shown on the time scale to the left of zero and the changes in surface potential due to exposure on the time scale to the right of zero. The intensity of exposure was about 15 lnxes respectively. The curve A in this figure represents a sensitive material comprising (cadmium sulfide +cadmium carbonate) base powder and it can be seen from the curve A that the surface potential of this sample begins to decay simultaneously with the commencement of exposure. The curve B represents a sensitive material comprising fine base powders containing cadmium iodide and its surface potential starts to decay rapidly after a lapse of about 0.6 to 0.8 second after commencement of exposure. The curve C represents a sensitive material comprising powders prepared by addition of 150 mg. of Crystal Violet to 150 g. of fine base powders containing cadmium iodide, which shows the sensitizing effect as well as the improvement of time lag in the commencement of surface potential decay.

The present invention prevents the deterioration of electrophotographic characteristics due to pre-exposure. It is a known phenomenon that where an electrophotographic sensitive material is radiated by means of intense light and charged directly thereafter at a dark place by corona discharge, its surface potential decays extremely rapidly in comparison with the case where no pre-exposure is carried out. This phenomenon is called the pre-exposure efiect, which is considered to be a kind of fatigue.

EXAMPLE 9 In respect to the electrophotographic sensitive material explained in the preceding example, pre-exposure effect was studied. That is, a sample of electrophotographic sensitive material was radiated for 2 minutes by means of a white fluorescent lamp, maintaining the intensity of illumination of the sample surface at 200 luxes, and after a lapse of 15 seconds in the dark, the electrophotographic sensitive material was charged negatively by a 7.0 kv. corona discharge and immediately thereafter (after about 2 seconds), its surface potential was measured. The surface potential at that time is designated here as LF(). Also, the electrophotographic sensitive material was exposed under the aforesaid pre-exposure condition and thereafter left for 5 minutes at a dark place, after which it was charged negatively under the same conditions and the surface potential was measured. This surface potential is designated as LF(300). Also, the electrophotographic sensitive material was left for 24 hours at a dark place for suflicient dark adaptation and then charged under the same condition without pro-exposure and its surface potential was measured. This surface potential is designated as DR. Generally, LF(5) is the lowest while LF(300) is higher, showing a certain recovery. Table 3 shows the surface potential ratios LF(5)/DR and LF(300)/DR obtained in this way.

TABLE 3 Cadmium iodide Cadmium iodide not contained contained LF (5) LF 300)] LF 5 LF 300 Coloring matter DR D R D if Rose Bengal 0. 05 0. 09 0.62 0. 75 osine Y 0. 03 0. 07 0. 61 0.93 Bromophenol blue 0. 19 0. 21 0. 12 0. 23 Crystal viol 0. 01 0. 09 0. 59 0.83 Malachite green- 0. 03 0. 08 0. 50 0. 71 Brilliant green... 0.06 0.28 0.76 0. 96 Rhodamine B, cone.... 0. 04 0. 09 0. 93 0. 92 Not added 0. 04 0. 07 0. 44 0. 70

As can be seen from Table 3, the addition of coloring matter does not always bring about improvement. The table shows that Bromophenol Blue has a sensitivitydecreasing eflect. Where cadmium iodide is not contained, the pre-exposure effect is improved whereas when cadmium iodide is contained, it is deteriorated. Crystal Violet, Malachite Green and Brilliant Green have a sensitizing effect and improve the pre-exposure eifect only when cadmium iodide is contained.

Although in the above examples the electrophotographic sensitive material has been negatively charged, it should be understood that it may alternatively be positively charged and adequate initial surface potential high sensitivity can be obtained which is comparable with the negatively charged material. The addition of dye will also provide the same desirable improvement for the positively charged material as the negatively charged material. That is where a sensitive material comprising base powders only was charged to a surface potential of +1000 v., its half-decay exposure time was about 4 seconds. In the case of a sensitive material composed of base powders containing cadmium iodide, it can be positively charged only to a maximum of some 200 v. In case where a sensitive material comprising base powders with Crystal violet added thereto was charged to a surface potential of 1000 v., its half-decay exposure time was about 6 seconds, with sensitivity being decreased due to 10 the addition of dyestuff. Further, in the case of a sensitive material composed of base powders containing cadmium iodide with the same dye added thereto, the time was about 0.5 second under the same condition and charging capacity as well as sensitivity was improved conspicuously.

Referring to and in addition to the values of n specified hereinbefore for the formula CdS-nCdCo the following ranges are felt to be preferable in the practice of the invention: (1) O n 5 which corresponds to the range specified hereinbefore in the Second Manufacturing Example of the photoconductive powder; (2) l n 5 which corresponds to the range stated in the Summary of Invention; and (3) O n 4.

Further, the operable range of the various dyestuffs mentioned hereinbefore is approximately 0.001% to 1% by weight of the photoconductive powder particles while the preferred range is 0.01% to 0.2% by weight of the photoconductive powder particles. The operable range of the iodide or iodine mentioned hereinbefore is 0.01 gram to 50 grams for each grams of the photoconductive powder particles while the preferred range is 1 gram to 30 grams for each 100 grams of the photoconductive powder particles. Also the size of the photoconductive powder particles may be 0.01 micron to 10 microns while preferably they are 0.05 micron to 3 microns.

What is claimed is:

1. A photoconductive material which comprises photoconductive powder particles of cadmium carbonate and cadmium sulfide having the formula CdS-nCdCO wherein O n 5, said cadmium carbonate and said cadmium sulfide being admixed with at least one dyestuif capable of absorbing radiation energy and transmitting it to said fine photoconductive powder particles, the amount by weight of said one dyestuff being substantially less than the amount by weight of said fine photoconductive powder particles.

2. The photoconductive material of claim 1 where lni 3. The photoconductive material of claim 1 where O n 4.

4. A photoconductive powder as in claim 1 where said one dyestuff is approximately 0.001% to 1.0% by weight of said fine photoconductive powder particles.

5. A photoconductive powder as in claim 4 where said one dyestutf is approximately 0.01% to 0.2% by weight of said fine photoconductive powder particles.

6. The photoconductive material of claim 1 wherein said dyestufit is selected from the group consisting of Eosine, Rose Bengal, Fluoresceine, Phloxine, ethyl Eosine, Malachite Green, Crystal Violet, Brilliant Green, Dicyanine, Kryptocyanine, Pinacyanol, Neocyanine, Merocyanines, Rhodamine B and Methylene Blue, said dyestuff being present in an amount of about 0.1% by weight of the said photoconductive powder particles.

7. The photosensitive material of claim 1 wherein iodine is contained within said fine photoconductive powder particles, the amount by weight of said iodine being less than the amount by weight of said fine photoconductive powder particles.

8. A photoconductive powder as in claim 7 where there are about 0.01 gram to 50 grams of said iodine for each 100 grams of said fine photoconductive powder particles.

9. A photoconductive powder as in claim 8 where there are about 1 gram to 30 grams of said iodine for each 100 grams of said fine photoconductive powder particles.

10. The photosensitive material of claim 1 wherein an iodine containing compound is contained within said fine photoconductive powder particles, said iodine compound being selected from the group consisting of lithium iodide, magnesium iodide, beryllium iodide, bismuth iodide, tungsten iodide, cesium iodide, strontium iodide, tin iodide, potassium iodide, cadmium iodide, antimony iodide, aluminum iodide, and zinc iodide, the amount by weight 1 1 of saidiodine containing compound being less than the amount by weight of said fine photoconductive particles.

11. A photoconductive powder as in claim 7 where there are about 0.01 gram to 50 grams of said iodine containing compound for each 100 grams of said fine photoconductive powder particles.

12. A photoconductive powder as in claim 8 where there are about 1 gram to 30 grams of said iodine containing compound for each 100 grams of said fine photoconductive powder particles.

13. The photoconductive material of claim 1 where the size of the photoconductive powder particles is approximately 0.01 micron to 10 microns.

14. The photoconductive material of claim 1 where the size of the photoconductive powder particles is approximately 0.05 micron to 3 microns.

15. A photoconductive material as in claim 1 where said dyestuif is approximately 0.001% to 1.0% by weight of said fine photoconductive powder particles and is selected from the group consisting of phthalein dyes, triphenylmethane dyes, cyanine dyes, Rhodamine B, and Methylene Blue.

16. A photoconductive material as in claim 15 where (1) said phthalein dyes are selected from the group con- 1'2 sisting of Eosine, Rose Bengal, Fluoresceine, Phloxine, and ethyl Eosine, (2) said triphenylmethane dyes are selected from the group consisting of Malachite Green, Crystal Violet, and Brilliant Green and (3) saidcyanine dyes are selected from the group consisting of Dicyanine, Krytocyanine, Pinacyanol, Neocyanine, and merocyanines.

References Cited UNITED STATES PATENTS 3,506,595 4/1970 Makino et a1. 252-501 3,541,028 11/ 1970 Makino et al. 252-501 3,121,006 2/1964 Middleton et al 96-15 3,052,540 9/1962 Greig 96-1.7 3,051,839 8/1962 Carlson et a1. 250-211 3,234,017 2/1966 Heyl et a1. 96-1 3,238,150 3/1966 Behringer et a1 252-501 1,730,505 10/ 1929 Hart 252-501 X 3,379,527 4/ 1968 Corrsin et al. 96-15 3,425,830 2/ 1969 Sanders 96-15 CHARLES E. VAN HORN, Primary Examiner US. Cl. X.R. 96-15; 252-501 Disclaimer 3,704,123.Katsu0 Malt-i120 and [um Swwato, Minami-Ashigara Machi, J a an. %ENS2III1Z91;2D11HTOCC1IU3UCTIVE MATERIAL. Patent a e 0v. 1sc almer e Ma 12 1972 10 th Fuji Photo Fiim 00., Ltd. y y e asslgnee Hereby disclaims the portion of the term of the ate t b Fe 1987 p 11. S11 sequent t0 LOflicz'aZ Gazette May $2 1973] 

